Endostatin, a 183 amino acid proteolytic cleavage fragment corresponding to the C-terminus of collagen 18, has been the subject of investigation by a number of laboratories because of its anti-tumor activity with no toxic side effects (O'Reilly et al. (1997) Cell, 88: 277-285.; Kisker et al. (2001) Cancer Res, 61:7669-7674; Dhanabal et al. (1999) Cancer Res, 59: 189-197; Yoon et al. (1999) Cancer Res, 59: 6251-6256; Folkman and Kalluri, (2003) Cancer Medicine, 6th edition, pp. 161-194. Hamilton: B. C. Decker Inc.). A number of anti-angiogenic activities have been reported for this protein, such as inhibition of endothelial cell proliferation, migration, and tube formation. Endostatin also suppresses vascular endothelial growth factor (VEGF)-induced vascular permeability (Takahashi et al. (2003) Faseb J, 17: 896-898). However, the mechanism of action of endostatin remains unknown. Endostatin inhibits endothelial cell migration by inhibiting phosphorylation of focal adhesion kinase via binding to α5β1 integrin (Wickstrom et al. (2002) Cancer Res, 62: 5580-5589). It also has been shown that cell surface glypicans are low-affinity endostatin receptors (Karumanchi et al. (2001) Mol Cell, 7: 811-822). Endostatin has been implicated in several signaling pathways, such as downregulation of c-myc (Shichiri and Hirata (2001) Faseb J, 15: 1044-1053), cyclin-D1 (Hanai et al. (2002) J Biol Chem, 277. 16464-16469) and RhoA activity (Wickstrom et al. (2003) J Biol Chem, 278: 37895-37901), blockage of VEGF signaling (Hajitou et al. (2002) Faseb J, 16: 1802-1804; Kim et al. (2002) J Biol Chem, 277: 27872-27879), and inhibition of the wnt-signaling pathway (Hanai et al. (2002) J Cell Biol, 158: 529-539). Furthermore, endostatin has been shown to bind and inactivate metalloproteinases (Kim et al. (2000) Cancer Res, 60: 5410-5413; Nyberg et al. (2003) J Biol Chem, 278: 22404-22411; Lee et al. (2002) FEBS Lett, 519: 147-152) and to regulate a spectrum of genes which suppress angiogenesis (Abdollahi et al. (2004) Mol Cell, 13: 649-663).
The crystal structures of both murine and human endostatin have been elucidated (Hohenester et al. (1998) Embo J, 17: 1656-1664; Ding et al. (1998) Proc Natl Acad Sci USA, 95: 10443-10448) and show a noncovalently held dimer at high concentration required for crystallization (Ding et al. (1998) Proc Natl Acad Sci USA, 95: 10443-10448). The presence of two disulfide bonds results in a highly folded structure. Endostatin binds one atom of zinc per monomer via the three histidines in the N-terminus of the molecule (histidines 1, 3, and 11) and asparatic 76. The heparin binding property of endostatin is mediated by noncontiguous arginines clustered over the three dimensional globular surface of the molecule (Sasaki et al. (1999) Embo J, 18: 6240-6248).
Oligomeric endostatin (NC1 and dimer) have been shown to be primarily associated with laminin in the basement membrane (Javaherian et al. (2002) J Biol Chem, 277: 45211-45218). This association may be important for some of the biological functions displayed by endostatin. On the other hand, the heparin binding properties of endostatin manifest themselves in its interaction with the cell surface. It is likely that endostatin has a number of biological functions mediated by different regions of the protein.